1. Field of the Invention
The present invention generally relates to a process for isomerization of cycloalkane, and more particularly to a process for isomerization of tetrahydrodicyclopentadiene (THDCPD) using a supported acidic ionic liquid as catalyst.
2. The Prior Arts
The acid catalysts have been used in petroleum and petrochemical industry for many years. For example the acid catalysts have been used in cracking, alkylation, isomerisation, polymerisation, etherification, sterification, amidation, and transesterification processes, and especially traditional Lewis acids (AlCl3 and ZnCl2) and Brönsted acids (HF and H2SO4) are often used as catalysts for alkylation and isomerisation. Although Lewis acids and Brönsted acids are effective in promoting the reaction, there are still a lot of problems to be overcome, such as product separation, catalyst recovery, facility corrosion, and treatment of spent acid catalyst and waste water. Solid acids or superacids have been investigated for the substitution of liquid acids, recently (Misono, M., CHEMTECH, November 23 (1993); Rao, P., Oil & Gas J., September 9, 56 (1996); Weitkamp, J., Catal. Today, 49, 193 (1999)).
One improved method is that anhydrous aluminum trichloride is directly supported on the inorganic carrier for the preparation of the supported Lewis acid catalysts, and such obtained supported Lewis acid catalysts exhibit high activity and selectivity for catalytic cracking of hydrocarbons, and the isomerization and alkylation of aromatic hydrocarbons (Getty, E. E., Inorg. Chem., 29, 1186 (1990); Hu, X., Appli. Catal., A: General, 217, 1 (2001). Drago et al. disclosed that it is preferable that the supported AlCl3 catalyst is prepared using carbon tetrachloride (CCl4) as a solvent (J. Am. Chem. Soc., 110, 3311 (1988)). Furthermore, the supported AlCl3 catalysts exhibit high activity and selectivity when montmorillonite (Clark, J. H., J. Chem. Soc. Chem. Commun., 56, 2037(1995)), polymer (Fuentes, G. A., J. Catal., 78, 436 (1982)), and molecular sieve [Jun, S., J. Catal., 195, 237 (2000); Hu, X., J. Catal., 195, 412 (2000)] are used as a carrier. Moreover, the application of the supported AlCl3 catalysts will become broader if the carrier selection, the catalyst supporting method, and the catalyst regeneration method can be improved ((Petrochemical Technology, 35(1), 88 (2006)).
Although the solid acid catalysts have many advantages, the acid sites in the solid acid catalysts are not easily accessible to reactants, and the proportion of the active sites is low. Furthermore, the catalyst activity is generally degraded at high reaction temperature due to the suppression of carbon deposit on the external surface of the catalyst. Therefore, the acidic ionic liquids are found to be a preferred alternative because of its adjustable properties, such as its acidity and solubility. Most of the ionic liquid catalysts are used in the liquid-liquid biphasic system, and thereby the reactants and the products are easily separated from the ionic liquid catalysts. The catalytic reaction can be effectively carried out due to the ionic liquid's non-volatile property and changeable acidity and solubility by changing the kinds of cation and anion and the molar ratio of cation and anion to aluminum trichloride. However, from the industrial point of view, the solid catalysts are considered to be the preferred ones because the products are easily separated and the fixed-bed reactor can be used. Although the liquid-liquid biphasic system has the advantages, a large amount of ionic liquid is required in use. In considering of economy and environmental protection, the supported ionic liquid systems are the preferred ones. The supported ionic liquid catalyst is in solid form, and however the active substance existing in ionic liquid supported on a solid surface. Therefore, the supported ionic liquid catalysts have the same reaction activity as that of the homogeneous catalysts.
The supported ionic liquid catalysts are prepared using the grafting or the impregnation methods, and in these two methods, the acidic ionic liquid is directly immobilized on SiO2, Al2O3, or MCM-41, as support. In another method, the cation-halide from the ionic liquid is covalently bonded to the resin, which then reacts with AlCl3 to form the supported acidic ionic liquid. In the other method, the —SiOH group on the silica formed by sol-gel technique is changed to —Si(OEt)3 group attached to the cation of the ionic liquid, and then AlCl3 is added to form the supported acidic ionic liquid (Journal of Petroleum, 41(4), 25(2005)). In most cases, BMIC-AlCl3 ionic liquid is loaded on the support powder by impregnation technique, and BMIC represents 1-methyl-3-butylimidazolium chloride, and in this method, the impregnation process is fast, but HCl will be formed after BMIC-AlCl3 ionic liquid reacts with the —OH group on the support surface, and the formed HCl will dissolve Al in BMIC-AlCl3(Castro, C. De, J. Catal. 196, 86 (2000)). However, this process suffer from major drawbacks: the action force between the ionic liquid and support is weak; and only partial of AlCl3 species existing in the ionic liquid loaded on the support are involved in the catalytic reaction due to the destruction of the support structure. However, the above-mentioned problems can be overcome by using grafting method. For example, 1-(triethoxy-silyl)-propyl-3-methyl-imidazolium chloride-AlCl3 reacts with the support in toluene by grafting technique to obtain the grafted acidic ionic liquid formed on the support (Volkenberg, M. H., Green Chem., 4, 88(2002); Sauvage, E., U.S. Pat. No. 6,969,693 B2 (2005); Kang, K. K., J. Chin. Inst. Chem. Engrs., 37(1), 17 (2006)). However, the —OH groups on the support can be first treated, and then the quaternary ammonium salt/AlCl3 ionic liquid is impregnated into the support. Consequently, HCl will not be formed when the ionic liquid containing AlCl3 is impregnated into the support,
In U.S. Pat. No. 3,381,046, the endo-tetrahydrodicyclopentadiene (endo-THDCPD) is isomerized to exo-tetrahydrodicyclopentadiene (exo-THDCPD) in the presence of sulfuric acid. In U.S. Pat. No. 4,086,284, the endo-tetrahydrodicyclopentadiene is isomerized to exo-THDCPD in the presence of aluminum trichloride. However, the ring-opening, cleavage, polymerization reaction will occur with sulfuric acid, and thereby there are a lot of problems left to be tackled, such as by-product formation, black coke, facility corrosion, and treatment of spent acid. When aluminum trichloride is used as a catalyst, a washing of the exo-tetrahydrodicyclopentadiene is required to remove any remaining aluminum trichloride, and consequently a large amount of sludge waste is produced. Adamantane (tricyclo[3.3.1.13,7]decane) can be synthesized using aluminum trichloride, superacid, or REY zeolite as a catalyst, and there are still a lot of problems left to be tackled, such as reaction selectivity, and facility corrosion.